Not Applicable
The invention is related to the field of network communications, and in particular to communications networks that route telephone calls via an Asynchronous Transmission Mode (ATM) or similar cell-based network.
There is an increasing trend toward convergence of two major activities, namely voice communication over telephone networks and data communications over data networks such as the Internet. Equipment and systems are being designed to handle both types of communications traffic, with specific pieces of the system being optimized for one or the other as necessary and other pieces being designed for both uses as necessary. Networks operating according to Asynchronous Transmission Mode (ATM) specifications are good examples of components that are tailored for use with both types of traffic. ATM is being deployed more and more extensively in large, wide-area communications networks to better handle the mixed nature of modern communications traffic.
As a result of the increasing use of ATM networks in wide-area communications network, there has been a need for equipment designed to interface with older, xe2x80x9clegacyxe2x80x9d telephone equipment and interfaces, on the one hand, and with an ATM network, on the other hand. For example, there is a large need for equipment that accepts the widely-used time-division-multiplexed (TDM) signals such as DS1, DS3, etc., from user equipment, and re-formats these signals to be carried as cells in an ATM network.
Traditional telephone switching equipment, much of which remains in widespread use, connects to other components in the long-distance network using xe2x80x9cinter-machine trunksxe2x80x9d or IMTs. An IMT is usually a TDM link such as a DS3 link, on which one or more ISDN Primary Rate Interfaces (PRIs) may be layered. Within each switch, a unique identifier is assigned for each IMT that links the switch with another switch in the network. The switch uses a Call Routing Table (CRT) to route incoming calls toward their destinations. The CRT contains a number of entries, each of which creates an association between some number of higher-order telephone number digits, such as the area code and perhaps exchange code, and the identifier for an IMT that should be used to carry calls to the destinations identified by the telephone number. For example, an entry might map the area code xe2x80x9c617xe2x80x9d to an identifier of a trunk that connects the switch with a switch that is along a route to Boston.
Because of the large installed base of traditional telephone switching equipment and software, there is a desire to design new equipment to be as xe2x80x9cbackward compatiblexe2x80x9d as possible, meaning that the equipment can interoperate with older equipment that uses long-existing functions and protocols, as well as with newer equipment that may use newer functions and protocols. Accordingly, there is a desire, for example, for switches and switch components that can translate between known call routing functions, such as described above, and newer networks and circuit-establishment protocols, such as are used in ATM networks.
The problem of routing voice communications over an ATM network has been addressed by the ATM Forum, an organization that coordinates ATM-related activities and promulgates specifications that describe standard ATM protocols and operations. Techniques related to xe2x80x9cVoice and Telephony over ATMxe2x80x9d, or VTOA, are described in ATM Forum specification AF-VTOA-0089.000 entitled xe2x80x9cVoice and Telephony over ATMxe2x80x94ATM Trunking Using AAL1 for Narrowband Servicesxe2x80x9d (version 1.0, July 1997), and a specification AF-VTOA-0078.000 entitled xe2x80x9cCircuit Emulation Service Interoperability Specificationxe2x80x9d (version 2.0, January 1997; generally referred to as xe2x80x9cCES-2xe2x80x9d).
The above-referenced ATM Forum documents describe various functions used in xe2x80x9cinterworkingxe2x80x9d equipment to provide the desired voice-over-ATM operation. The general concept is to use an ATM Virtual Connection (VC) to carry multiple calls by assigning different octets of an ATM Adaptation Layer 1 (AAL1) payload to the different calls. Thus for example the first octet may be a sample from a DS0 circuit carrying call #1, the second octet a sample from a DS0 carrying call #2, etc.
One peculiarity in the ATM Forum scheme is that there is an uneven mapping from the octets of an AAL1 payload to the octets of ATM cells. That is, an AAL1 payload can be of variable size and can, in fact, span multiple ATM cells. A technique that uses a pointer embedded in certain AAL1 messages is used to identify payload boundaries. One aspect of this technique is that the octets designated to carry samples for a given telephone call change from cell to cell in a pattern that generally repeats over some number of cells, the number being determined by the size of the AAL1 payload. In general, the octets of successive AAL1 messages that carry a given call are referred to as a xe2x80x9ccell slotxe2x80x9d. Because of the unevenness introduced by the use of AAL1, this nomenclature is somewhat misleading, because the octets for a given call do not always appear in the same octet of an ATM cell. However, the term xe2x80x9ccell slotxe2x80x9d is widely used, and is understood to refer to the set of octets from successive cells that carry a given call, despite the varying of the positions of the octets over successive ATM cells.
While the above-described ATM Forum techniques go far in providing a mechanism for carrying voice over an ATM network, shortcomings nonetheless exist. The above-referenced VTOA specifications do not address the problem of interfacing to legacy routing software used in existing telephone switches. Moreover, the techniques are limited in the manner in which individual telephone calls can be assigned to VCs and cell slots. It would be desirable to achieve voice-over-ATM operation in a manner that can be readily deployed with existing telephone components, such as legacy telephone routing software, and that provides maximum flexibility in order to achieve efficient and robust system performance.
In accordance with the present invention, methods and apparatus for provisioning inter-machine trunks in a communications system that uses an ATM long-haul network are disclosed. A significant degree of backwards compatibility with existing telephone switch functions is achieved, and techniques are used that enhance efficient use of resources in the ATM network.
An inter-machine trunk (IMT) is established between two switches through a cell-based network, such as an ATM network. The IMT is configurable to carry a number of virtual connections (VCs) each capable of carrying at least one call. The number of VCs in the IMT is based on provisioning information generated by a network operator in communication with one of the switches.
When a call request is received at a telephony interface of a switch, such as at a DS3/PRI or similar trunk interface, it is first determined whether the call should be routed via the IMT. This determination is made using a call routing table that associates destination telephone numbers with trunks via which users having those telephone numbers can be reached. If the IMT is to be used, a VC of the IMT is selected to carry the call. A call request message is forwarded from the switch to a switch at the other end of the IMT. The call request message identifies the called user and the VC selected to carry the call. Each switch then establishes a connection between the selected VC of the IMT and a respective circuit of the telephony interface. For the originating switch, this is the interface circuit to the calling party; for the destination switch, it is the interface circuit to the called party. This switch-to-switch connection is then used for the telephone call.
Generally, one switch controls the establishment of the IMT by executing multiple repetitions of a signaling process with the other switch, each repetition resulting in the creation of one VC.
Each virtual connection includes a plurality of cell slots that may be allocated for carrying different calls through the cell-based network. Thus, a cell slot is also chosen, in addition to a VC, for carrying the call. The identity of the cell slot is also signaled in the call request message forwarded to the destination switch. In one embodiment, each VC/cell slot combination is uniquely associated with a corresponding Trunk Circuit Identifier Code (TCIC). A VC and cell slot are selected by selecting the associated TCIC, and the signaling is accomplished by including the TCIC in the call request message forwarded to the destination switch. Each switch then establishes a connection between the allocated VC/cell slot and the respective telephony interface circuit.
The IMT may be one of two types, either a pre-established IMT referred to as a xe2x80x9cPIMTxe2x80x9d or an on-demand IMT referred to as a xe2x80x9cVIMTxe2x80x9d. A PIMT is a quasi-permanent connection that is set up and torn down by action of an operator. The PIMT provides a minimum guaranteed bandwidth or data-carrying capacity at the cost of some inefficiency. The VIMT is created automatically when a call is received and no previously-established connections are available to carry the call. The VIMT promotes efficient utilization of the ATM network, because it consumes bandwidth and VCs only when needed. In many cases, it is desirable to configure an IMT using both a PIMT and a VIMT, with connections on the PIMT being used before any connections on the VIMT are used.
The use of standard identifiers, such as TCICs, to identify IMTs in the ATM network enables the technique to be used with existing call-routing procedures in wide use. The ability to assign a call to any of several VCs, and any of several cell slots within a VC, provides desirable flexibility. Further, the use of PIMTs to guarantee available trunk bandwidth, and VIMTs to provide bandwidth on demand, enhances the efficient use of the resources of the ATM network.
Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.